The present invention relates to lead-free solder alloy compositions. More specifically, the present invention relates to lead free solder alloy compositions that provide a direct substitute of Snxe2x80x94Pb solder used in electronic assemblies.
Snxe2x80x94Pb solder, with a eutectic composition Sn-37Pb (e.g., 63 wt % Sn, 37 wt % Pb) or near eutectic composition (e.g., 60 wt % Sn-40 wt % Pb), has a eutectic melting temperature of 183xc2x0 C. As one of the primary components of solders, lead (Pb) reduces the melting point of tin (Sn), increases its strength, improves it ductility, and provides excellent thermal cycling fatigue resistance of the solder. In addition to these technical advantages, lead is a readily available and low cost metal. Snxe2x80x94Pb solders are therefore widely used in electronic assemblies throughout the world.
Lead-free soldering is driven by increased concerns about the impact of lead on health and the environment. In the United States, the electronics manufacturing industry has come to a consensus view as to the ultimate abandonment of tin-lead solders reflected in the Lead Exposure Act (S.729) and the Lead Tax Act (H.R. 2479, S. 1347). Starting Jan. 1, 2004, European nations will be requiring the use of lead-free solder alloys in all electronic assemblies. In Japan, similar legislation is proposed that will prohibit lead from being sent to land fills and other waste disposal sites.
In response to the lead-free soldering issue, massive research efforts worldwide have been carried out to identify a suitable substitute. The work is generally targeted to the development of a direct substitute for Sn-/37Pb solder for surface mount technology (SMT) manufacturing. Since a solder with higher melting temperature will have a major impact on the other polymeric materials used in microelectronic assembly and encapsulation, an acceptable substitute should offer a melting point around 183xc2x0 C. and possess eutectic properties. The desired features of a lead-free alternative to Pb/Sn eutectic in the assembly include: lowest melting temperature, minimal freezing range, ease of manufacture, ease of recycling, minimum materials cost and compatibility of suitable flux with a No Clean process.
A recent review by Abtew et al. in Mat. Sci. and Eng. 27(2000)95-141 revealed that approximately 70 Pb-free solder alloys have been proposed so far by a combination of researchers and manufacturers. The majority of the alloys are based on Sn, In and Bi, with Sn as matrix metal. Other alloying elements are Zn, Ag, Sb, Cu and Mg. The Sn rich compositions are considered to be most likely candidates. The alloys investigated by some organizations are listed in Table 1.
Note: Alloy compositions are given in the form xe2x80x9cSnAg2.5Cu0.8Sb0.5,xe2x80x9d which means: 2.5% Ag, 0.8% Cu, and 0.5% Sb (weight percent), with the leading element (in this case, Sn) making up the balance to 100%.
Many lead free solder alloys have been patented for electronic applications. For example, U.S. Pat. No. 5,730,932 to Sarkhel, et al., suggests certain solder alloys containing Sn, Bi, In and Ag. Also, U.S. Pat. No. 5,328,660 to Gonya, et al., suggests a quaternary solder alloy of 78.4% Sn, 2% Ag, 9.8% Bi and 9.8% In (weight percentage). In U.S. Pat. No. 4,806,309 to Tulman, a tin base lead-free solder composition containing Bi, Ag, and Sb is proposed. In U.S. Pat. No. 5,344,607 to Gonya, et al., a Sn rich ternary solder alloy containing Sn, Bi and In is disclosed. Moreover, U.S. Pat. No. 6,231,691 to Anderson, et al., provides a Snxe2x80x94Agxe2x80x94Cu alloy modified by a low level of element Ni and Fe.
Snxe2x80x94Znxe2x80x94Bi solders are disclosed in U.S. Pat. No. 5,942,185 to Nakatsuka et al., U.S. Pat. No. 6,334,905 to Hanawa et al. and Taiwanese Pat. No. TW431931. Ternary solders comprising Snxe2x80x94Znxe2x80x94Bi as main components are hopeful from the point of melting temperature. In order to prevent Zn oxidation of Snxe2x80x94Znxe2x80x94Bixe2x80x94Agxe2x80x94Cuxe2x80x94In solder, addition of less than 1% P is disclosed in U.S. Pat. No. 6,241,942 to Murata et al. U.S. Pat. No. 6,228,322 to Takeda et al. claims that Sm, Ga or a mixture of these elements with other rare earth elements can be added to Snxe2x80x94Ag alloy or Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu alloys to enhance the mechanical strength of lead-free solder. Lead-free tin alloys comprising In, Al, Mg and Zn are provided in the world patent WO 98/32886.
It is found that, among 67 lead-free alloys compositions published, there are 9 alloys that have eutectic melting temperatures close to that of Snxe2x80x94Pb solder. However, the major components of these alloys are comprised of the elements bismuth and indium. These alloys are not considered to be a real alternative of Snxe2x80x94Pb solder for the following reasons: a) the price of indium is high; b) large amounts of bismuth and indium tend to lead to low melting phases formed in the system, which have a bad influence on the reliability of the solder pad, and raise concerns about thermal fatigue at higher temperature; c) it becomes difficult to recover usably purified materials from the solder alloy for recycling use when bismuth or indium is used as an additive element of the solder.
On the other hand, lead-free alloys based on Snxe2x80x94Ag, Snxe2x80x94Cu and Snxe2x80x94Agxe2x80x94Cu eutectic systems have melting points in the 217 to 227xc2x0 C. range, which is significantly higher than that of 63Sn37Pb. These alloys are thus not a suitable direct substitute of conventional Snxe2x80x94Pb solder (e.g., Sn-37Pb).
As indicated by Abtew et al., in Mat. Sci. and Eng. 27(2000)95-141 there is believed to be no single alloy that can be simply xe2x80x9cdropped inxe2x80x9d as one-for-one replacement for Snxe2x80x94Pb solder. The alloy systems investigated thus far are limited, and a more wide-ranging investigation is necessary.
Potential lead-free alloys with high percentage of tin reported by the Litton company are listed in Table 2.
As shown above, the lead-free solder alloy selection continues to be the research subject of many works. Recently, the thermodynamic equilibrium calculation has become one of the effective theoretical tools in identification of lead-free solder. For example, a preliminary calculation of the ternary phase diagram for Snxe2x80x94Agxe2x80x94Cu as described by Miller et al in xe2x80x9cA Viable Tin-Lead Solder Substitute: Snxe2x80x94Agxe2x80x94Cuxe2x80x9d, Journal of Electronic Materials, July 1994, Volume 23, Number 07, p.595-602, indicated the occurrence of a ternary eutectic reaction at 217.4xc2x0 C. for a composition of Sn-3.8Ag-2.3Cu(wt. %) using existing binary alloy thermodynamic and phase equilibrium without ternary interaction parameters. The results are in excellent agreement with the experiments. Furthermore, the surface tensions of Sn-based solder alloys have been predicted successfully by using the Butler equation.
In view of the foregoing, a lead free solder alloy composition that provides a direct substitute of conventional Snxe2x80x94Pb solder used in electronic assemblies is desired.
The invention relates, in one embodiment, to a lead free solder composition. The lead free solder composition includes Sn, Ag, and Mg, and has a melting temperature under 200xc2x0 C. In most cases, the composition includes between about 3.0 to about 14.0 wt % Ag and about 1.9 to about 3.2 wt % Mg. In other cases, the melting temperature is close to the melting temperature (e.g., 183xc2x0 C.) of conventional leaded solders.
The invention relates, in another embodiment, to an electronic structure. The electronic structure includes a first electronic component that is electrically bonded to a second electronic component by a solder alloy consisting essentially of 3.0 to about 14.0 wt % Ag and about 1.9 to about 3.2 wt % Mg and balance Sn.
The invention relates, in another embodiment, to a method of joining two electronic components. The method includes providing a first and a second electronic component to be joined. The method also includes connecting the components with a solder consisting essentially of about 3.0 to about 14.0 wt % Ag and about 1.9 to about 3.2 wt % Mg and balance Sn.
The invention relates, in another embodiment, to a lead free solder composition. The lead free solder composition includes Sn, Zn, and Mg. In most cases, the composition has a property similar to a corresponding property of a conventional Snxe2x80x94Pb solder (e.g., melting temperature, mushy range, surface tension, wetting ability and the like).